The present invention relates to a digital signal modulation apparatus, and more particularly, to a differentially encoding quadrature phase shift keying (DEQPSK) modulation apparatus and method thereof, capable of simplifying circuitry and improving modulation speed.
For digital data transmission, a binary waveform is superposed onto a carrier. Data transmission methods such as amplitude modulation (AM), phase modulation (PM) and frequency modulation (FM) are chiefly used. Also used frequently is a system involving the mixture of AM modulation and PM modulation such as a quadrature amplitude modulation (QAM) method.
A signal processed by a binary phase shift keying (BPSK) modulation method is one having a fixed amplitude. When the data is placed in a certain level, the data has a fixed phase, while when the data is placed in a different level, the phase of the data is shifted by 180.degree.. Actually, a BPSK-modulated signal is produced by applying a waveform as a digital time oscillator carrier and a baseband signal as a modulation waveform, to a balanced modulator.
Further, differential phase shift keying (DPSK) modulation and differentially encoded phase shift keying (DEPSK) modulation are variations of the BPSK modulation method, which have the added advantage of removing the uncertainty as to whether the demodulated data has been inverted. Particularly advantageous, DPSK modulation does not require a sync carrier which is necessary for the demodulation of a received BPSK modulation signal. The DEPSK modulation method has a further advantage in that it does not require a delay unit which is necessary in the DPSK modulation apparatus.
Also, the quadrature phase shift keying (QPSK) modulation method is one for performing phase shifting in proportion to a value or an input symbol, in which only half of the bandwidth (in comparison with BPSK modulation) is necessary for the transmission of one bit of data.
FIG. 1 is a graphical diagram of constellation of a modulation signal in the QPSK modulation method, and shows that a phase difference of 90.degree. is generated in the QPSK modulation method. On the other hand, a phase difference of 180.degree. is generated in the BPSK modulation method.
The DEQPSK modulation method is one type of QPSK modulation, wherein modulation noise generated due to a sudden change of the phase value (or large differences therein) is reduced by varying the degree of phase shift generated between symbols. Here, the phase shift is .+-..pi./4 or .+-..pi./4 which are both integer multiples or .pi./4.
FIG. 2 is a graphical diagram of constellation of a modulation signal in the DEQPSK modulation method.
FIG. 3 is a block diagram of a conventional DEQPSK modulation apparatus.
The DEQPSK modulation apparatus of FIG. 3 comprises a serial-to-parallel converter 31 for converting a serially input modulation signal train b.sub.m into two binary signal trains, a differential phase encoder 32 for encoding the converted signal as two signals of in-phase (I) and quadrature (Q) channels, a digital-to-analog converter 33 for converting signals I.sub.K and Q.sub.K obtained by differential phase encoder 32 into two analog signals, a baseband filter 34 for baseband-filtering the I- and Q-channel signals, a phase shifter 35 for shifting the phase of a carrier fc by 90.degree. (.pi./2), first and second multipliers 36 and 37 for respectively multiplying the outputs of baseband filter 34 with the two phase-shifted signals, and a mixer 38 for mixing the signals supplied from multipliers 36 and 37 to output a combined signal X(t).
In the operation according to the above construction, serial-to-parallel converter 31 converts the signal train b.sub.m into the two signal trains, and outputs the converted signals. Differential phase encoder 32 encodes the above converted signals as I- and Q-channel signals (I.sub.K and Q.sub.K), and outputs the encoded signals. The I- and Q-channel signals are converted into two analog signals via digital-to-analog converter 33. Thereafter, the analog signals are each baseband-pass-filtered via baseband filter 34, and the thus filtered signals are respectively multiplied with the carrier signal fc and the phase-shifted carrier in first and second multipliers 36 and 37. Then, the multiplied signals are combined through mixer 38 to complete the modulating process.
Here, as represented in Table 1, the modulation signal train b.sub.m is converted into the binary signal trains of odd-numbered bit train X.sub.K and even-numbered bit train Y.sub.K, starting from the first bit of the signal train. Then, the combination of the two signal trains represents a phase difference .DELTA..phi..
TABLE 1 ______________________________________ X.sub.K Y.sub.K .DELTA..phi. ______________________________________ 1 1 -3.pi./4 0 1 3.pi./4 0 0 .pi./4 1 0 -.pi./4 ______________________________________
The I- and Q-channel signals I.sub.K and Q.sub.K are represented as the following equations, as accomplished by differential phase encoder 32: EQU I.sub.K ={I.sub.K -cos[.DELTA..phi.(X.sub.K,Y.sub.K)]}-{Q.sub.K -sin[.DELTA..phi.(X.sub.K,Y.sub.K)]} EQU Q.sub.K ={I.sub.K -sin[.DELTA..phi.(X.sub.K,Y.sub.K)]}-{Q.sub.K -cos[.DELTA..phi.(X.sub.K,Y.sub.K)]}
wherein I.sub.K and Q.sub.K are the current values of the I- and Q-channels, and I.sub.K-1 and Q.sub.K-1 are the values at the preceding pulse.
The differentially encoded final phase value .phi..sub.K becomes a result of adding the phase variation value according to odd-numbered bit train X.sub.K and even-numbered bit train Y.sub.K to the phase value at the time of the preceding pulse. The differentially encoded final phase value .phi..sub.K is represented thus: EQU .phi..sub.K =.phi..sub.K-1 +.DELTA..phi.
As described above, the conventional DEQPSK modulation method requires considerable calculation to obtain values I.sub.K and Q.sub.K, and has a complicated structure in terms of hardware. Accordingly, embodiment of the conventional DEQPSK modulator as a single chip is very difficult due to the difficulty in minimizing its circuitry.